[数据结构与算法]Leetcode 17. 电话号码的字母组合

思路
设置字母和数字映射字典，设置backtrack函数，参数index表示层数，用来遍历digits的（题目中给出数字字符串）

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        answers = []
        answer = []      
        letter_map = {
            '2': 'abc',
            '3': 'def',
            '4': 'ghi',
            '5': 'jkl',
            '6': 'mno',
            '7': 'pqrs',
            '8': 'tuv',
            '9': 'wxyz'
        }
        def backtrack(digits, index):
            if index == len(digits):
                temp_str = ""
                for temp in answer:
                    temp_str += temp

                answers.append(temp_str)
                return
            letters = letter_map[digits[index]]
            for letter in letters:
                answer.append(letter)
                backtrack(digits, index+1)
                answer.pop()

        if not digits: return []
        backtrack(digits, 0)
        return answers